In automobile seats, these armatures make up the frame of the seating or back, to which are fixed the suspension elements, or lap, the whole being covered by the padding. The link, commonly hinged, between the back and the seating thus takes place between their respective armatures, by means of a hinge and position locking part to adjust the inclination of the back. Thus conventionally, a back armature has the general shape of an upside down "U"; the ends of the uprights making up the arms of this U are welded to said hinge parts, and the crossbar joining these two uprights at the top of the back has bushings for fixing the headrest.
Two types of armatures are currently used: those made of stamped metal sheet or plate and those made of tubes. Plate armatures usually consist of two or three parts that are prestamped and assembled by welding so as to make up said U.
Plate armatures have the advantage that they can be shaped, during the stamping, so as to provide the best possible mechanical strength. As a result, they have a good strength to weight ratio and, furthermore, the bends of the U can be shaped with a virtually unlimited radius of curvature. But the stamping operation, which requires specific tools for each armature shape, and the assembly of the stamped parts make these plate armatures costly.
Tubular armatures are less costly, since the tubes used are round tubes made of common steel, commonly available, which can be bent by means of conventional bending tools so as to give them the desired U-shape. But this type of armature has several disadvantages. Since the cross-section of the tube used is necessarily constant, the bending strength, which must be especially high towards the bottom of the back, can only be increased by increasing the cross-section of the tube, in diameter or thickness, throughout its entire length. This results in a relative oversizing of the parts under least stress, and in an increase of the weight, and the bending radius of the tube cannot be reduced as much as possibly desired.
In addition, regardless of the type of armature used, holes are pierced in the armature to attach the lap to it. To ensure a more adequate lumbar support, the fastenings of the lap, and therefore the holes, are closer to one another in the area located above the hinge, which is specifically the area where the bending stresses of the armature are the highest. In a seat with a plate armature, the weakening resulting from these closely arranged holes can be compensated by a reinforcement of the armature in this area, by adjusting the shape of the stamped plate. In a seat with a tube armature, this is not possible, except by adding reinforcement parts.